This invention relates to nuclear magnetic resonance (NMR) methods. More specifically, this invention relates to improved NMR imaging methods which eliminate the effects of spurious free induction decay (FID) NMR signals caused by imperfect 180.degree. radio frequency (RF) pulses.
NMR imaging methods utilize a combination of pulsed magnetic field gradients and pulsed RF magnetic fields to obtain NMR imaging information from nuclear spins situated in a selected region of an imaging sample. The imaging sample is typically positioned in a static magnetic field B.sub.o. The effect of field B.sub.o is to polarize nuclear spins having net magnetic moments so that a greater number of the spins align with the field and add to produce a net magnetization M. Individual polarized nuclear spins, and hence magnetization M, resonate (or precess about the axis of field B.sub.o) at a frequency .omega. given by the equation EQU .omega.=.gamma.B.sub.o (1)
in which .gamma. is the gyromagnetic ratio (constant for each NMR isotope).
As will be more fully described hereinafter, magnetic field gradients are necessary to encode spatial information into the NMR signal. If a magnetic field gradient along an imaging volume is a function of position, then so is the frequency .omega.. In fact, if the imaging gradient is linear, the frequency spectrum is a one-dimensional projection of the NMR signal distribution along the direction of the gradient.
RF magnetic field pulses are directed orthogonal to the B.sub.o field used to excite nuclear spins to resonance. The frequency of the RF pulse needed to induce resonance is the same as the resonance frequency defined by equation (1). Two types of RF magnetic field pulses commonly used are 90.degree. and 180.degree. pulses. A 90.degree. RF pulse causes magnetization M to rotate 90.degree. about the axis defined by the applied RF magnetic field vector in a frame of reference rotating at the resonant frequency .omega. about the direction of field B.sub.o, relative to the laboratory frame of reference. Thus, if the direction of field B.sub.o is assumed to be the positive Z-axis direction of a Cartesian coordinate system, a 90.degree. RF pulse will cause magnetization M along B.sub.o to rotate into the transverse plane defined by the X-and Y-axis, for example. Similarly, a 180.degree. RF pulse causes magnetization M along B.sub.o to rotate 180.degree. about the axis of field B.sub.o (e.g., from the positive Z-axis direction to the negative Z-axis direction).
Nuclear spins rotated 90.degree. into the transverse plane, or through some other angle such that magnetization M has a vectorial component in the transverse plane, will produce an FID NMR signal which is observable upon termination of RF excitation. The FID signal can be detected by a receiver coil positioned to be sensitive along the transverse plane.
An NMR FID signal will not be observed if the nuclear spins are inverted 180.degree. from the direction of the static magnetic field B.sub.o, because magnetization M under these conditions does not have a component in the plane of the receiver coil. While this is true for ideal 180.degree. RF pulses, in practice the 180.degree. pulses are rarely ideal, and in virtually all cases a small spurious FID occurs immediately following the 180.degree. pulse. The FID can arise because the 180.degree. RF pulse is not precisely 180.degree.. If it were to be set at 160.degree., for example, then there could be spurious FID signals arising from previously unexcited spins which would simulate the application of a 20.degree. RF pulse. In some cases, the RF transmitter coils used to irradiate the imaging sample generate inhomogeneous fields so that parts of the imaging sample do not receive precisely a 180.degree. pulse and therefore contribute an FID component in the transverse plane. Some NMR techniques utilize selective 180.degree. RF pulses to invert nuclear spins by 180.degree. in a planar section of an imaging sample, while leaving those spins outside the section substantially unaffected. In this case, regions bordering the planar section of nuclear spins selectively inverted by a 180.degree. RF pulse can actually experience a 90.degree. RF pulse and thereby generate a large FID.
The effect of imperfections in 180.degree. RF pulses on NMR images can be quite severe. If the spurious FID signal lasts sufficiently long it will add to the desired NMR signal that contains the spatial encoding information for imaging. Because the spurious FID signal has different spatial encoding, it generates artifacts in the reconstructed image. In the case of selective 180.degree. pulses, the spurious signal can render the selective 180.degree. RF pulses unusable.
The NMR pulse sequences in accordance with the present invention eliminate the effects of spurious FID NMR signals caused by imperfect 180.degree. RF pulses. Either the 90.degree. pulses or the 180.degree. pulses may be phase alternated so that the spurious FID signals cancel when the desired signals are analyzed. Although the invention is described with reference to NMR imaging methods, its applicability is not limited thereto. The invention is applicable to other NMR methods in which spurious FID signals, caused by imperfect 180.degree. RF pulses, produce undesirable effects. One such method is the use of selective 180.degree. RF pulses in localized NMR spectroscopy. Another is the use of selective 180.degree. RF pulses in localized blood flow measurement. The invention is also applicable to three-dimensional NMR imaging methods, such as those described and claimed in the commonly assigned application Ser. No. 365,229 filed Apr. 5, 1982 by the same inventors as herein and which is hereby incorporated by reference as background material.